Have mg_adaptor skip over off-lined devices.

Previously the mg_adaptor will keep trying to read from off-lined
PDUs in the hope that once they were brought on-line data could be
collected from them at the earliest possible moment. However,
practically it will take human being more time to fixup off-lined
PDUs so such behaviour will do no extra good but leaving logs lots
of useless messages about the off-lined PDUs.

If a PDU is marked as off-lined, the producer thread won't update
its status to oBIX server. Now have the consumer thread also stop
reading from it if it is off-lined.

Signed-off-by: Qingtao Cao <harry.cao@nextdc.com>